Common techniques for fabricating displays and semiconductor electronic devices involve several imaging steps. Typically, in each step, a substrate coated with a resist or other sensitive material is exposed to radiation through a photo-tool mask to effect some change. Each step has a finite risk of failure. The possibility of failure at each step reduces the overall process yield and increases the cost of the finished article.
A specific example is the fabrication of color filters for flat panel displays such as liquid crystal displays. Color filter fabrication can be a very expensive process because of the high cost of materials and low process yield. Traditional photolithographic processing involves applying color resist materials to a substrate using a coating technique such as spin-coating, slit and spin or spin-less coating. The material is then exposed via a photo-tool mask and developed.
Thermal transfer processes have been proposed for use in the fabrication of displays and in particular color filters. In such processes, a color filter substrate also known as a receiver element, is overlaid with a donor element that is then image-wise exposed to selectively transfer a colorant from the donor element to the receiver element. Preferred methods of image-wise use laser beams to induce transfer of the colorant to the receiver element. Diode lasers are particularly preferred for their ease of modulation, low cost and small size.
Thermal transfer processes can include laser induced “thermal transfer” processes, laser-induced “dye transfer” processes, laser-induced “melt transfer” processes, laser-induced “ablation transfer” processes, and laser-induced “mass transfer” processes. Colorants transferred during thermal transfer process can include suitable dye or pigment based compositions. Additional elements such as one or more binders may be transferred, as is known in laser-induced mass transfer processes.
Direct imaging systems typically employ hundreds of individually modulated beams in parallel to reduce the time taken to complete images. Imaging heads with large numbers of such “channels” are readily available. For example, one model of SQUAREspot® thermal imaging head manufactured by Kodak Graphic Communications Canada Company, British Columbia, Canada has several hundred independent imaging channels, each channel having power in excess of 25 mW. The array of imaging channels can be controlled such that an image is written in a series of swaths which are closely abutted to form a continuous image.
One problem with multi-channel imaging systems is that it is extremely difficult to ensure that all channels have identical imaging characteristics. Different imaging characteristics among channels may result from differences in the output radiation that the channels project upon the imaged media. Variations in the output radiation emitted by the array of imaging channels may originate from channel-to-channel variations in power, beam size, beam shape and/or focus. These variations contribute to the production of a common imaging artifact known as banding. Banding is often particularly prominent in the area between two successively-imaged swaths. This is primarily because the end of the last imaged swath and the beginning of the next imaged swath are usually written by channels at opposite ends of a multi-channel array. As such, these channels are more likely to have differing imaging characteristics. A gradual increase in a spot characteristic from channel-to-channel may or may not be visible within the swath itself, but when a swath is abutted with another swath, a visible discontinuity at the swath boundary may result in a pronounced artifact in the image. Banding can be a function of any overlap or separation of successive swaths as well as channel variance within each of the respective swaths.
Various approaches have been used in an attempt to precisely position swaths next to one another. Precise control over the positions of imaged swaths is typically necessary but not sufficient to eliminate banding, especially when the imaging system changes over time in response to varying environmental factors. Banding artifacts may not be solely attributable to the imaging system. The imaged media itself may also contribute to banding, and other imaging artifacts.
U.S. Pat. Nos. 4,900,130; 5,164,742; 5,278,578; 5,808,655; 6,597,388; 6,765,604; and 6,900,826 disclose various methods to attempt to alleviate various artifact problems such as banding.
“Raster scan line” interleaving techniques have been proposed to reduce banding and other imaging artifacts. Examples of raster scan line interleaving techniques are disclosed in U.S. Pat. Nos. 5,691,759; 6,597,388; 6,784,912; and 6,037,962. Image artifacts including banding may be further aggravated when a pattern of non-contiguous features is imaged.
Image artifact complications can also arise when a thermal transfer process is employed in the imaging of a repeating pattern of non-contiguous features as typically required in the production of color filters. Color filters typically consist of a repeating pattern of color elements, each of the elements corresponding to one of the colors required by the color filter. Each of the color elements is typically smaller in width than the width of the overall swath that can be imaged with a multi-channel imaging head. Various image artifacts including banding can result when varying color transfer efficiency causes differences between the color elements, as well as within the elements themselves. Since the lines form a repeating pattern, a visual beating readily perceptible by the human eye results which typically reduces the quality of the color filter.
There remains a need for imaging methods that lessen the visibility of banding and other imaging artifacts associated with the imaging of patterns of non-contiguous features. There remains a need for imaging methods that lessen the visibility of banding and other imaging artifacts associated with the imaging of repeating patterns of non-contiguous features such as the patterns of color elements in color filters.